In recent years, with the progress of digitization of television program contents and the spread of distribution services of motion pictures on the Internet, there is an increasing demand that a large volume of data be communicated on a network. Accordingly, the Ethernet (registered trademark and the same shall apply hereinafter) is widely used in which high-speed communication can be realized inexpensively. In transmitting a large volume of user data, such as IP (Internet Protocol) packets, a plurality of frames are transmitted continuously (in a bursty manner) in the Ethernet. In a network in which such bursty traffics are multiplexed, frame discard occurs because data exceeding the wire rate are sometimes inputted. Accordingly, in order to ensure QoS (Quality of Service), it is necessary to reduce degree of burst by performing peak rate shaping at the entrance of a network. In this case, because a frame for each user flow is transmitted, in the shaper, at a timing of a frame interval in accordance with the set bandwidth, it is required that the frame is stored in the buffer for every user flow. To realize this, two methods are generally known: one is a method in which a physical buffer is shared by a plurality of users (shared buffer method: JP-A-Hei10 (1998)-32581); and the other is a method in which a physical buffer is virtually divided to be allocated to each flow (linked list method: JP-A-Hei07 (1995)-221762).